This invention relates to the processing of forged titanium articles to improve the microstructure of such articles.
High strength titanium alloys are widely used in aerospace applications. Considerable research has been directed toward increasing strength and fatigue properties of titanium alloy airframe components.
Due to the nature of titanium transformation and alloying stabilization behavior, titanium grades can be grouped into three major classes, depending on the phase or phases present in their microstructures. These are alpha/near-alpha, alpha+beta, and near-beta/beta types.
Most titanium alloys currently used for high performance aerospace applications are alpha+beta (e.g., Ti-6Al-4V) and near-alpha (e.g., Ti-6Al-2Sn-4Zr 2Mo) alloys. Commercial emphasis for the manufacture of these alloy forgings has been largely placed on the alpha+beta processings to assure adequate strength and ductility. Alpha+beta alloys are the most commonly used titanium alloys and are designed for intermediate strength and high fracture resistance in both airframe and engine applications. Near-alpha alloys possess excellent high temperature properties and are generally designed for high creep properties at high temperatures. Because of lack of toughness in the solution treated and aged condition and relatively poor hardenability, alpha+beta alloys have commonly been used in the annealed condition. As a result, the strength capability of titanium alloys cannot be effectively utilized.
Forging of near-alpha or alpha+beta titanium alloys is one of the most common methods for producing high integrity components for fatigue-critical airframe and gas turbine engine applications. Currently, forging of these classes of alloys is done at temperatures below the beta transus temperature of the alloys because the microstructures developed have a good combination of tensile and fatigue properties. On the other hand, forging near or above the beta transus temperature provides certain advantages in terms of reduced press load and much better shape definition, since the alloy plastic flow resistance is greatly reduced. Unfortunately, the microstructure developed as a result of such forging is a lenticular beta microstructure which is inferior in terms of fatigue performance.
What is desired is a method for forging near-alpha or alpha+beta titanium alloys which will reduce press load and provide better shape definition, thereby reducing cost, and which will provide forgings having a fatigue-resistant microstructure.
Accordingly, it is an object of the present invention to provide an improved process for forging near-alpha and alpha+beta titanium alloy components.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.